The present invention generally relates to noise reducing systems for video signals, and more particularly to a noise reducing system for reducing noise in a video signal which is recorded onto a magnetic recording medium and reproduced from the magnetic recording medium, by subjecting the recording video signal to a pre-emphasis with respect to the vertical direction of a picture in a recording system at the time of the recording and by subjecting the reproduced video signal to a de-emphasis with respect to the vertical direction of the picture in a reproducing system at the time of the reproduction.
Conventionally, pre-emphasis and de-emphasis circuits are known for reducing noise in a video signal which is recorded and reproduced. In a recording system, the recording video signal is subjected to a pre-emphasis in the pre-emphasis circuit, and the reproduced video signal is subjected to a de-emphasis in the de-emphasis circuit within a reproducing system. In the case of a frequency modulated video signal, the signal-to-noise ratio becomes poorer for high-frequency components of the frequency modulated video signal. However, the pre-emphasis circuit can emphasize the high-frequency components of the video signal before the recording. At the time of the reproduction, the high-frequency components of the reproduced video signal are de-emphasized back to their original levels. Therefore, it is possible to record and reproduce the high-frequency components of the video signal with a satisfactory S/N ratio by use of such pre-emphasis and de-emphasis circuits.
A conventional pre-emphasis circuit employed a highpass filter or a transversal filter comprising a capacitor (C) and a resistor (R). A conventional de-emphasis circuit employed a lowpass filter or a transversal filter comprising a capacitor (C) and a resistor (R). Both the pre-emphasis and de-emphasis circuits were designed to add, to an information which is related to a signal obtained at the present point in time, a weighed information which is related to a signal obtained at a past point in time which is extremely close to the present. This weighted information is obtained by subjecting the information which is related to the signal obtained at the past point in time which is extremely close to the present, to a predetermined weighting.
However, as will be described later on in the specification in conjunction with the drawings, the conventional emphasis circuits were designed to perform the pre-emphasis or the de-emphasis, by subtracting from or adding to an information related to a predetermined point in a reproduced picture, information related to a plurality of points which are positionally lagging (to the left of the predetermined point in the reproduced picture) or advanced (to the right of the predetermined point in the reproduced picture) with respect to the predetermined point on a horizontal scanning line which includes the predetermined point. According to such emphasis circuits, the high-frequency noise in the horizontal direction will be averaged, and the S/N ratio of the video signal will be improved. In the present specification, the emphasis performed in the conventional emphasis circuits will be referred to as horizontal emphasis.
Because the conventional emphasis circuit only performs the horizontal emphasis described above, the noise can be reduced satisfactorily in a case where the noise is in the form of a vertical line which is long in the vertical direction of the picture and has a short width in the horizontal direction of the picture, for example. The noise can be reduced satisfactorily in this case, since the frequency of the noise in the horizontal direction is high. On the other hand, in a case where the noise is in the form of a horizontal line which has a short width in the vertical direction of the picture and is long in the horizontal direction of the picture, the frequency of the noise in the horizontal direction is low. For this reason, the noise reducing effect was very poor in this case, and the conventional emphasis circuit suffered a disadvantage in that such a noise in the form of a horizontal line could hardly be reduced. In the present specification, the above noise in the form of a horizontal line which has a low frequency in the horizontal direction of the picture and has a high frequency in the vertical direction of the picture, will be referred to as a horizontal line noise. In actual practice, the horizontal line noise is easily generated in the reproduced picture. Hence, there was a demand for a system which could effectively reduce such a horizontal line noise.
When a pre-emphasis is performed in the pre-emphasis circuit, an overshoot and an undershoot occurs at the leading and trailing edges of the output pre-emphasized signal. When this pre-emphasized signal is frequency-modulated in a frequency modulator, the instantaneous frequency of the frequency modulated signal which is produced from the frequency modulator becomes extremely high at the leading edge of the pre-emphasized signal where the overshoot occurs, and the level rises to white level from black level. Thus, when the above frequency modulated signal is recorded onto and reproduced from a magnetic tape, the level of the frequency modulated signal may exceed a slicing range of a limiter which is located in a stage prior to a frequency modulator, in a case where the overshoot exceeds a predetermined level. When the level of the frequency modulated signal exceeds the slicing range of the limiter, a signal dropout will occur in the output of the limiter, and this signal dropout will be frequency-demodulated as a low-frequency signal in the frequency demodulator. As a result, the level of the frequency demodulated signal will drop to the black level, and the so-called color inversion phenomenon is introduced.
Accordingly, a clipping circuit which clips the emphasized signal so that the level at the tip of the overshoot does not exceed a predetermined level, was generally provided in a stage prior to the frequency modulator. However, as the degree of emphasis was increased in order to obtain a larger emphasis effect, the overshoots and undershoots in the emphasized signal became greater. In this case, the degree of clipping became greater so as to properly clip the greater overshoots and undershoots, and as a result, the picture quality became poor at the leading and trailing edges of the emphasized signal. For this reason, it is impossible to excessively increase the degree of emphasis.
Therefore, it is impossible to reduce the horizontal line noise by increasing the degree of emphasis, and it is highly desirable to realize a system for reducing such horizontal line noise.
Hence, the present inventors have previously proposed a noise reducing system for video signals, in which the disadvantages described heretofore have been eliminated. This previously proposed system is disclosed in a U.S. patent application Ser. No. 576,155 entitled "NOISE REDUCING SYSTEM FOR VIDEO SIGNAL" filed Feb. 2, 1984. According to this previously proposed system, an emphasis is performed with respect to the vertical direction of the picture, and a de-emphasis complementary to the emphasis is performed with respect to the vertical direction of the picture. According to this previously proposed system, it is possible to satisfactorily reduce especially the horizontal line noise, since the emphasis and the de-emphasis are performed with respect to the vertical direction of the picture. In the present specification, such an emphasis performed with respect to the vertical direction of the picture, will simply be referred to as a vertical emphasis as opposed to the horizontal emphasis described before.
However, when the input video signal having a large level is subjected to the same pre-emphasis as the input video signal having a small level, overshoots and undershoots of large levels will occur. These overshoots and undershoots of large levels, will be clipped in the clipping circuit which is located in a stage subsequent to the pre-emphasis circuit, and the original information which is missing due to the clipping inevitably increases. In this case, there is a problem in that the signal waveform of the output video signal of the de-emphasis circuit at the time of the reproduction, becomes different from the signal waveform of the original video signal. In addition, especially when the input video signal having the large level is subjected to the same pre-emphasis or de-emphasis as the input video signal having the small level, there is a problem in that the signal waveform at the edge of the video signal becomes greatly distorted.